In the field of automobiles, domestic electric appliances and the like is known a component having a closed cross-section structure formed by shaping two parts separately and joining these parts to each other. Also, hydroforming or roll forming is known as a method of producing a closed cross-section structural component with a curved form along its longitudinal direction.
In the conventional hydroforming method, it is necessary to weld all of peripheral edge portions before the pouring of a machining fluid. JP-A-2008-119723 discloses a hydroforming machine, a hydroforming method and a hydroformed product, in which deep drawn products having an excellent sealing property in bulging can be obtained from two or more metal sheets without lap-welding all peripheral edge portions and the production efficiency capable of simultaneously shaping plural components is excellent.
JP-A-2000-263169 discloses a method of producing a closed cross-section curved long material comprising a roll forming step of shaping a band plate into nearly a closed cross-section with multistage forming rollers, joining butt portions thereof with a caulking roller and curving the resulting closed cross-section long material with many bending rollers along a moving direction of the band plate.
JP-A-2003-311329 discloses a technique capable of obtaining a pressed product with a distortion on the way of a closed cross-section form from a raw material in which a high-quality closed cross-section pressed product having a light weight and a high-rigidity distorted portion is provided at a low cost.
JP-A-2011-062713 discloses a method of producing a closed cross-section structural component having a curved form along its longitudinal direction through press forming by joining two folded steel sheets each having a curved form at their both flange portions to each other and deforming to move the flange portions close to each other.
However, the hydroforming method disclosed in JP '723 and the roll forming method disclosed in JP '169 have problems that the production rate is slow and equipment cost is high compared to the press forming. Also, the press forming method disclosed in JP '329 has a problem that it is difficult to perform butting of the end faces in a component having a curved form in its longitudinal wall portion. Further, the method disclosed in JP '713 has a problem that there is a limitation in the weight reduction because it is required to join flange portions of two press formed steel sheets to each other by welding.
It could therefore be helpful to provide a method of producing a polygonal closed cross-section structural component with a curved form along its longitudinal direction that is capable of reducing the weight of a product at a low cost only by press forming, and a polygonal closed cross-section structural component produced by the method.